1. Field of the Invention
The present invention relates to an image forming optical system which is useful for both, slimming (thinning) of an optical system and achieving improved performance, and an electronic image pickup apparatus using such image forming optical system.
2. Description of the Related Art
A digital camera has reached a level of being used practically, regarding making large the number of pixels (high image quality), and small-sizing and slimming. Also from a function point of view and a market point of view, a digital camera has substituted a silver salt 35 mm film camera. Therefore, improvement in optical specifications such as a high zooming ratio of an optical system, a wide angle, and a large aperture ratio, as well as further increase in the number of pixels with the same small-size and slimness, have been sought strongly as a next trend of evolution.
For instance, as a zooming optical system which has hitherto been used as favorable for high zooming ratio, an optical system disclosed in Japanese Patent Application Laid-open Publication No. 2003-255228 is available. In Japanese Patent Application Laid-open Publication No. 2003-255228, a so-called positive-lead zooming optical system in which, an initial lens group (a first lens group), in order from an object side, has a positive refractive power, has been disclosed.
Moreover, when an attempt is made to realize small-sizing and slimming of a high-magnification zooming optical system by using a similar zooming optical system, a correction of chromatic aberration is susceptible to be a problem.
Here, the chromatic aberration will be described in detail.
The chromatic aberration includes a paraxial chromatic aberration and a high-order chromatic aberration. Moreover, The paraxial chromatic aberration includes a chromatic aberration of a focal-point position and a chromatic aberration of a focal length. The correction of such chromatic aberrations has been achieved normally, by coinciding of the focal point position and the focal length for two predetermined wavelengths, (such as, for a C-line (656.27 nm) and for an F-line (486.13 nm). This state is called as a state in which, an achromatism of the C-line and the F-line has been possible. This can be achieved by selecting an appropriate Abbe's number for a plurality of lenses in an optical system.
However, for the other wavelengths which differ from the two predetermined wavelengths, the focal-point position and the focal length do not coincide. Or in other words, in most of the cases, an aberration remains. The chromatic aberration which has remained is called as a secondary spectrum. An optical characteristic which has an effect on an amount remained is a partial dispersion ratio.
In a case of a normal optical material, an optical material with a low dispersion (high Abbe's number) has a relatively small partial dispersion ratio, and an optical material with a high dispersion (low Abbe's number) has a large partial dispersion ratio. Consequently, a low-dispersion optical material having the partial dispersion ratio as large as possible, and a high-dispersion optical material having the partial dispersion ratio as small as possible are to be selected. Accordingly, it is possible to reduce the secondary spectrum.
Whereas, the chromatic aberration of high-order corresponds to a high-order component (a distortion of color) for an image height of a spherical aberration of color (chromatic spherical aberration), a chromatic coma, and a chromatic aberration of magnification. In these aberrations, dispersion of the material and a source of origin of the high-order aberration for a reference wavelength (for example, 587.56 nm for d-line) are intertwined intricately. Generally, for correcting these, correction of aberration for the reference wavelength and correction of paraxial chromatic aberration are to be sacrificed. Consequently, it is desirable to correct independently.
As a technology for correcting the chromatic aberration, optical systems disclosed in Japanese Patent Application Laid-open Publication Nos. 2005-316047 and 2005-352265 have been known. In Japanese Patent Application Laid-open Publication Nos. 2005-316047 and 2005-352265, a transparent medium having an effective dispersion characteristic or a partial dispersion characteristic which a conventional glass does not have, has been introduced in the optical system.
Furthermore, in an electronic image pickup apparatus in which, an electronic image pickup element has been used, a flare due to the chromatic aberration for an h-line (404.66 nm) is susceptible to occur. As a patent literature in which an importance of correction of the chromatic aberration for h-line has been described, Japanese Patent Application Laid-open Publication No. 2001-208964 has been known.